Communication devices such as User Equipments (UEs) are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two UEs, between a UE and a regular telephone and/or between a UE and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
UEs may further be referred to as wireless terminals, mobile terminals and/or mobile stations, mobile telephones, cellular telephones, laptops, tablet computers or surf plates with wireless capability, just to mention some further examples. The UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a network node. A cell is the geographical area where radio coverage is provided by the network node.
The network node may further control several transmission points, e.g. having Radio Units (RRUs). A cell can thus comprise one or more network nodes each controlling one or more transmission/reception points. A transmission point, also referred to as a transmission/reception point, is an entity that transmits and/or receives radio signals. The entity has a position in space, e.g. an antenna. A network node is an entity that controls one or more transmission points. The network node may e.g. be a base station such as a Radio Base Station (RBS), eNB, eNodeB, NodeB, B node, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size.
Further, each network node may support one or several communication technologies. The network nodes communicate over the air interface operating on radio frequencies with the UEs within range of the network node. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the UE to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks. In LTE the cellular communication network is also referred to as E-UTRAN.
An E-UTRAN cell is defined by certain signals which are broadcasted from the eNB. These signals contain information about the cell which can be used by UEs in order to connect to the network through the cell. The signals comprise reference and synchronization signals which the UE uses to find frame timing and physical cell identification as well as system information which comprises parameters relevant for the whole cell.
A UE needing to connect to the network must thus first detect for a suitable cell, as defined in 3GPP TS 36.304 v11.5.0. This is performed by measuring on received reference signals sent by neighboring cells, also referred to as “listening” for a suitable cell. The suitable cell is commonly the cell with best quality of signal. Listening for a suitable cell may comprise searching for synchronization signals transmitted from the network node in an OFDM subframe. When a suitable cell is found the UE performs random access, according to a system information for the cell. This is done in order to transmit a Radio Resource Control (RRC) connection setup request to the network node. Assuming the random access procedure succeeds and the network node receives the request, the network node will either answer with an RRC connection setup message, which acknowledges the UEs request and tells it to move into RRC connected state, or an RRC connection reject, which tells the UE that it may not connect to the cell. In RRC connected state the parameters necessary for communication between the network node and the UE are known to both entities and a data transfer between the two entities is enabled.
To facilitate handover to other cells, each network node may store cell identities that are supported by the other network nodes in an address database, in order to know how to contact the network node of potential target cells for handover. Each network node serving a cell typically stores in the data base which cells it has neighbor relations to, i.e. which of the cells in the area UEs often perform handover to. The cell's neighbor relations will hereafter be referred to as the cell's “neighbor relation list”.
Cell specific Reference Signals (CRS) are UE known symbols that are inserted in a Resource Element (RE) of a subframe of an Orthogonal Frequency-Division Multiplexing (OFDM) time and frequency grid and broadcasted by the network node. Each RE has an extension in the frequency domain corresponding to an OFDM sub carrier and an extension in the time-domain corresponding to an OFDM symbol interval.
The CRS are used by the UE for downlink channel estimation. Channel estimation is used for demodulation of downlink data both when the UE is in RRC connected state and is receiving user data and when the UE is in RRC idle state and is reading system information. Due to the latter use case, the CRSs must be transmitted even from cells which do not have any UEs in RRC connected state since the eNB cannot know whether a UE wants to access the network until it performs random access. Downlink cell specific reference signals are inserted within the first and third last OFDM symbol of each slot with a frequency domain spacing of six sub-carriers. A slot is a time period of the OFDM time and frequency grid, which is usually 0.5 msec long. A problem with the known technology is therefore that cells without any UEs in RRC connected state still consume power due to CRS broadcasting.
In case several antennas are used by the network node for transmitting and each antenna is representing a cell, each antenna has to transmit a unique reference signal in order for the UE to connect to that specific cell. When one antenna transmits, the other antennas have to be silent in order not to interfere with the first antennas reference signal. To reduce the interference of reference signals between the cells, the position of the CRS is usually shifted in frequency between the cells. The CRS can be shifted between 0-5 sub carriers, each sub carrier corresponding to a frequency shift of 15 kHz for LTE. The cell specific frequency shift can be derived from the physical Cell Identity (Cell ID) which is signaled to the UE by selection of appropriate Primary Synchronization Channel (PSCH) and Secondary Synchronization Channel (SSCH
Although this solution reduces the interference of reference symbols between cells, it has the problem that the reference symbols of one cell will disturb Physical Downlink Shared Channel (PDSCH) and Physical Downlink Control Channel (PDCCH) symbols of neighboring cells.
Hence, even though cells do not have any UEs in RRC connected state, disturbance may impact UE DL throughput in neighboring cells. This will especially be the case when the UE is in and/or close to borders between cells.
Reducing the power of the CRS may mitigate this problem. However, in order to access a cell the UE must be able to hear the CRS of the cell, i.e. the UE must be able to recognize and receive the CRS transmitted from the cell. Therefore reducing the power of the CRS also shrinks the size of the cell, since more distant UEs no longer will hear the CRS. Furthermore, the quality of the channel estimates used for demodulation decreases when the Signal to Interference Ratio (SINR) on the CRS decreases. Reducing the power of the CRS therefore causes degradation of cell edge performance. This degradation is further aggravated when the load in the network increases, especially if the data is transmitted with higher power than the CRS, which is usually the case when the effect of CRS interference is to be reduced.